CODEWITHSUNDEEP
c++vectorstlsizedeque
taigi100
taigi100

Reputation: 2839

Difference in size between std::vector and std::deque

Right after I declare a vector<int> and a deque<int> if I print out sizeof on both of them, the std::vector has 12 bytes ( I guess begin, end and size ) while deque has 40 bytes. Where do those extra bytes come from?

I'm using Code::Blocks IDE 13.12 and I did select the C++11 standard to be used.

Upvotes: 2

Views: 875

Answers (3)

Mike Seymour
Mike Seymour

Reputation: 254771

The size is implementation dependent; but it's not surprising that the control structure for a more complicated structure like deque might be larger than a simple one like vector (which, as you say, can be easily managed by three pointers, or a pointer and two sizes).

Looking at the GNU implementation, it stores precalculated "start" and "end" iterators, presumably because they're difficult to calculate on demand, and easier to update when the container changes. Each of those is quite complicated, containing four pointers: the current position, the start and end of the current block, and a pointer to a map structure needed to move between blocks. The deque also has a (possibly redundant?) pointer to that map, and the size (again, presumably difficult to calculate on demand), giving a total of ten pointers/sizes, as you observed.

Upvotes: 2

Bathsheba
Bathsheba

Reputation: 234885

sizeof does not give you the number of elements in a container; rather the compile time evaluated size of the structure.

So the actual value is down to the implementation of the standard library and, as such, can vary from platform to platform.

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409482

The sizeof operator takes the size of the object and not the size of its contents.

So if an object have three data member variables, the sizeof operator will get the size of those member variables, but if one of them is a pointer to more data then you only get the size of the pointer and not the length of the data it points to.

Upvotes: 1

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